The present invention relates to devices for the operational checking of the quality of digital radio links by periodic evaluation of the parity of sequences of a number of pre-determined bits.
It is known to check the quality of transmission of a digital train without redundency by calculation of the parity of sequences of n successive transmitted bits. Thus, a parity information is obtained every n bits which is transmitted to the receiver of the link where the parity is calculated on the same sequences and the result is compared with the parity information received. This permits an evaluation of the transmission quality in the form of an error rate which serves as a criterion, particularly for bringing about the switching of equipment when this rate is above a predetermined value.
This requires the transmission of a time reference determining the beginning of the sequence for the parity calculation, so that it relates to the same sequences on transmission and reception. To this end supplementary bits are inserted in the transmitted digital train, and the parity bit is transmitted with the information. In general, advantage is taken of this modification in the signal pattern to include information necessary for operation in the same pattern.
However, this arrangement provides a needlessly frequent error rate information for high binary flow rates. For example for the flow rate of 34 Mbits/sec. the transmission or a parity bit every 10.sup.4 bits, corresponding to the detection of a threshold error rate of 10.sup.-4, represents the transmission of a parity information every 300 .mu.s, whilst the switching operating time of equipment which can be controlled by such an information requires approximately 40 ms.
Above all this arrangement leads to a change to the in line binary flow rate and the use of numerous supplementary components in the equipment, which causes an increase in their consumption, cost and a decrease in their reliability, both with respect to the terminal stations and the relay stations.